The family of G-protein coupled receptors (GPCRs) is one of the most important classes of proteins from both functional and structural standpoints. The human genome contains nearly 950 genes coding for GPCRs, of which nearly 450 genes have been implicated as therapeutic targets. Ligand binding to GPCRs induces multiple receptor conformations and different ligands may stabilize different receptor conformations (Kenakin & Miller, 2010, Pharmacol. Rev. 62(2):265-304). The concept of functional selectivity is based on the hypothesis that different receptor conformations recruit different signaling proteins which leads to preferential activation of one signaling pathway over another (Mailman, 2007, Trends Pharmacol. Sci. 28(8):390-396). In addition to selecting the signaling pathways, agonist-induced receptor conformations can also potentially affect receptor signaling properties.
Among the GPCRs, the subfamily of dopamine receptors has attracted attention from biologists and pharmacologists. In the central nervous system, dopamine receptors are widely expressed and involved in the control of locomotion, cognition, emotion and neuroendocrine secretion. In the peripheral system, dopamine receptors are present more prominently in kidney, vasculature and pituitary, where they affect mainly sodium homeostasis, vascular tone, and hormone secretion. While there are numerous examples of functionally-selective ligands, which preferentially activates one signaling cascade but not others, functionally-selective ligands that alter receptor signaling properties are rare and have not been described for dopamine receptors.
The neurotransmitter dopamine controls a wide variety of physiological and behavioral functions in mammals via five major subtypes of dopamine receptors. They are broadly classified into the “D1 like” and “D2 like” dopamine receptors based on pharmacology and function. The D1-like consists of D1 and D5 receptors, while the D2-like consists of D2, D3, and D4 receptors. The D3 receptor primarily couples to the pertussis toxin-sensitive Gα-proteins (Gi/Go) (Ahlgren-Beckendorf & Levant, 2004, J. Recept. Signal Transduct. Res. 24(3):117-130). When transfected into different cell lines, the D3 receptor couples to adenylyl cyclase V isoform (Robinson & Caron, 1997, Mol. Pharmacol. 52:508-514) and initiates signaling events including phosphorylation of mitogen-activated protein (MAP) kinases (Cussac et al., 1999, Mol. Pharmacol. 56(5):1025-103). D2 and D3 dopamine receptors also modulate potassium and calcium channel function (Seabrook et al., 1994, Br. J. Pharmacol. 111:391-393; Werner et al., 1996, Mol. Pharmacol. 49:656-661). Transfected D3 receptors couple robustly to natively expressed G-protein coupled inward rectifier potassium (GIRK) and voltage-gated P/Q type calcium channels, and inhibit firing of spontaneous action potentials and secretory activity in the AtT-20 neuroendocrine cell line (Kuzhikandathil & Oxford, 1999, J. Neurosci. 19(5):1698-1707; Kuzhikandathil & Oxford, 2000, J. Gen. Physiol. 115:697-706; Kuzhikandathil et al., 1998, Mol. Cell. Neurosci. 12:390-402). The D3 receptor further couples to natively expressed adenylyl cyclase V (Kuzhikandathil & Bartoszyk, 2006, Neuropharm. 51:873-884), MAP kinases (Westrich & Kuzhikandathil, 2007, Biochim. Biophys. Acta-MCR 1773:1747-1758) and ion channels (Kuzhikandathil & Oxford, 1999, J. Neurosci. 19(5):1698-1707; Kuzhikandathil & Oxford, 2000, J. Gen. Physiol. 115:697-706; Kuzhikandathil et al., 1998, Mol. Cell. Neurosci. 12:390-402; Kuzhikandathil et al., 2004, Mol. Cell. Neurosci. 26:144-155) in AtT-20 cells.
The expression of D3 dopamine receptor is altered under many pathological conditions and following chronic treatment. In Parkinson's disease, levodopa-induced dyskinesias are associated with a specific up regulation of D3R expression in putamen and globus pallidus internal segment, regions that normally express the D2 receptor (Bezard et al., 2003, Nat. Med. 9(6):762-767; Guigoni et al., 2005, Parkinsonism Related Disorders 11 Suppl 1, S25-29). In rodent models, the behavioral sensitization associated with levodopa treatment is mediated by upregulated D3 receptors (Guillin et al., 2001, Nature 411(6833):86-89). In schizophrenia, D3 receptor expression levels are increased two fold in the basal ganglia. Antipsychotic treatment has also been reported to change the expression of D3 receptor. The density of D3 receptor is increased in chronic cocaine users in striatum and substantia nigra, as well as in the nucleus accumbens. Stress and depression-induced down regulation of D3 receptor expression is reversed following chronic antidepressant treatment.
Dopamine receptors are targets for the treatment of various neurological and psychiatric disorders, such as Parkinson's disease, schizophrenia, drug addiction, depression, bipolar disorder, attention deficit hyperactivity syndrome, Tourette's syndrome, Huntington's disease and migraine.
Parkinson's disease (also known as Parkinson disease) is a degenerative disorder of the central nervous system that often impairs the sufferer's motor skills, speech, and other functions (Jankovic, 2008, J. Neurol. Neurosurg. Psychiatr. 79(4):368-76). Parkinson's Disease is characterized by muscle rigidity, tremor, a slowing of physical movement (bradykinesia) and a loss of physical movement (akinesia) in extreme cases. The primary symptoms of Parkinson's Disease are the results of decreased stimulation of the motor cortex by the basal ganglia, normally caused by insufficient formation and action of dopamine, which is produced in the dopaminergic neurons of the brain (specifically the substantia nigra). Secondary symptoms may include high level cognitive dysfunction and subtle language problems. Parkinson's Disease is both chronic and progressive. At present, there is no cure for Parkinson's Disease, but medications may provide relief from the symptoms.
The most widely used form of treatment is L-dopa (levodopa). Levodopa is transformed into dopamine by L-aromatic amino acid decarboxylase (also known as dopa-decarboxylase) in the dopaminergic neurons. However, only 1-5% of levodopa enters the dopaminergic neurons. The remaining levodopa is often metabolized to dopamine elsewhere, causing a wide variety of side effects (“Symptomatic pharmacological therapy in Parkinson's disease”. Parkinson's Disease. London: Royal College of Physicians, 2006, pp. 59-100). Due to feedback inhibition, levodopa administration results in a reduction of the endogenous formation of levodopa, and so eventually becomes counterproductive. Levodopa may also be co-administered with carbidopa ((2S)-3-(3,4-dihydroxyphenyl)-2-hydrazino-2-methylpropanoic acid), which prevents levodopa metabolism elsewhere in the body. The dopamine agonists bromocriptine, pergolide, pramipexole, ropinirole, piribedil, cabergoline, apomorphine, and lisuride have been found to be moderately effective.
Levodopa-induced dyskinesia (LID) is a particularly serious side effect of the long-term use of levodopa. These motor fluctuations occur in more than half of Parkinson's Disease patients after 5-10 years of levodopa treatment, with the percentage of affected patients increasing over time, and LID is thought to be potentially irreversible. Dyskinesia most commonly occurs at the time of peak levodopa plasma concentrations and is thus referred to as peak-dose dyskinesia. As patients advance, they may evidence diphasic dyskinesia, which occurs when the drug concentration rises or falls. Attempts to moderate dyskinesia by the use of other treatments, such as bromocriptine (Parlodel™), appear to be ineffective. In order to avoid dyskinesia, patients with the young-onset form of the disease are often hesitant to commence levodopa therapy until absolutely necessary for fear of suffering severe dyskinesia later on. Currently, there is no pharmacotherapeutic means of treating LID in patients suffering from Parkinson's Disease.
Interestingly, there is an alteration of dopamine receptor expression in most disorders associated with the dopaminergic system, such as Parkinson's Disease. Changes in dopamine receptor expression are also observed following chronic treatment of these neurological and psychiatric disorders. In the case of D3 dopamine receptor, changes in expression have been reported in Parkinson's disease, schizophrenia, depression, and drug addiction. Following chronic drug treatment, studies have reported an upregulation of D3 receptor in LID in Parkinson's disease and antipsychotic-induced tardive dyskinesia in schizophrenia.
The ability of using levodopa as a therapeutic agent in the treatment of Parkinson's Disease is severely hampered by the likelihood that levodopa-induced dyskinesia (LID) will eventually develop. There is a need in the art for novel therapeutic agents that treat, ameliorate or prevent levodopa-induced dyskinesia in patients suffering from Parkinson's Disease. The present invention fulfills this need.